A digital subscriber line (xDSL) is a high-speed data transmission technology for transmission on an unshielded twisted pair (UTP) cable. Except a digital subscriber line for baseband transmission, an xDSL for passband transmission enables the xDSL and a traditional telephone service to coexist on a same pair of twisted pair cables by using a frequency division multiplexing technology. The xDSL occupies a high frequency band, the traditional telephone service occupies a baseband below 4 kilohertz (KHz), and a traditional telephone service signal is separated from an xDSL signal by using a splitter. The xDSL for the passband transmission is modulated by using discrete multitone. A system for providing multiple xDSL access is referred to as a DSL access multiplexer (DSLAM). As shown in FIG. 1 and FIG. 2 multiple channels of signals that access the DSLAM on lines between central office (CO) end devices and customer premises equipment (CPE) interfere with each other due to the principle of electromagnetic induction, which is referred to as crosstalk.
Energy of both near-end crosstalk (NEXT) and far-end crosstalk (FEXT) is enhanced as a frequency band increases. Because DSL spectrum resources are limited, and a longer line indicates larger attenuation, a resource of a higher spectrum cannot improve performance. In an OSD system, because NEXT crosstalk is generated when upstream and downstream spectrums are overlapped, an xDSL line or service is affected by the NEXT crosstalk more severely when a wider frequency band is used.
Although a vectoring technology in existing G.993.5 standards cancels FEXT crosstalk, an NEXT crosstalk problem in an OSD system cannot be resolved. Consequently, performance of the OSD system sharply degrades. To reduce impact from the NEXT crosstalk, NEXT and FEXT joint cancellation needs to be used to cancel the NEXT and FEXT crosstalk, or a DSM level 2 (L2) dynamic spectrum management technology is used to restrain the NEXT crosstalk. A prerequisite for using these technologies is to estimate an NEXT crosstalk channel.
In the prior art, a crosstalk channel estimation method includes sending a downstream probing signal and receiving an NEXT crosstalk signal in an upstream direction within an upstream silent period so that the NEXT crosstalk channel is obtained through measurement. Similar operations are performed in a downstream direction. A disadvantage of the crosstalk channel estimation method is that when a signal is also sent in an uplink direction, an uplink received signal includes both the FEXT crosstalk and the NEXT crosstalk that cannot be separated. Therefore, there is a relatively large error in measuring a crosstalk channel.